Gnome Monosoupape Cutaway

Recently, while reading a couple of autobiographies by World War One fighter piltos, I became increasingly  entranced by the engineering of the machines they flew. In particular by the curious, but omnipresent GNOME rotary engine that features  in the designs of so many of the great aircraft of the era, including the Sopwith Camel.

Radial engines are one thing, I could understand the principle: The engine stays but and the prop’ rotates, Everything is fed into that engine in a more or less conventional way but with a rotary the entire engine spins, fixed to the prop. So how on earth does it recieve fuel and sparks? And where are the exhaust pipes? It was a head scratcher. But thanks to the internet and you tube I finally understood and what a clever device that Gnome, especially the Monospupape, really was!

A rotary does not need a flywheel as it’s own spinning does the same job, it also cools itself very easily even when sitting on the ground not moving  and in the case of he Monospoupape version it doesn’t even have a conventional carbutettor or throttle system, it just runs flat out , regulated only by flicking the ignition on and off to  slow it down for landing. So the entire thing is very light in weight, something that was vital to the designs of the era where a heavy engine was the last thing your flimsy canvas-and-piano wire airframe needed!

Monosoupape refers to ‘one valve’ and thats were things get really original. The one valve at the top of each cylinder acts as both exhaust and inlet. How? Well it really works like a 2 stroke with the fuel being fed into the crank case and entering the cylinder by transfer ports uncovered in the cyclinder walls as the piston reaches the bottom of it’s stroke.

Avro 504A B with 110hp Gnome engine under the cowling

The clever bit is that the valve has been open for what would be almost 2 strokes (“blow and suck” as the old text books would term it) on a conventional design and having exhausted the burnt gases on it’s upstroke, sucked in a lungfull of fresh air on it’s way back down . At the last moment the fuel is drawn in via the ports as the valve closes and the next but is like any other 4 stroke engine “squeeze -bang-blow” .

Exhaust gases go straight out of the port ito the fresh air. No exhaust manifolding required, but herein lies one of the engines inherent problems.

The exhaust gas also contains a large amount of oil as the engine runs on a sort of 2-stroke mix of petrol and castor oil and a total-loss system. Clean it wasn’t. Sopwith Camels and their kin ran with a metal covering round the top two-thirds of the engine to dust the spent gas below the fuseage rather than letting it blow stright back into the cockpit. Castol oil being what it is, a couple of hours sat in a mist of the stuff was apt to make the pilots of the most ‘regular’ of troops!

The entire engine is such an all-in-one unitary construction it could be bolted onto all manner of aircraft designs, and it was! By the time World War 1

started it was already among the most plentiful aero engines designs available and over time it was enlarged from the original 7 cylinder 80hp through successive variations to a 9 cylinder 180hp version by the end of hostlities.   Such was it’s effectivenesss, Tommy Sopwith himself dewcribed it as “one of the greatest single advances in aviation”.

You could even couple several together in the fashion of the 1913 Radley-England Waterplane (seen below) Imagine that lot all spinning away at full chat !

Triple-Gnome set up from 1913 Radley-England Waterplane.

The Gnome wasn’t the only rotary of this type, W O Bentley built one for the RFC towardsthe end of the war and in Frence the La Rhone company, which produced a rotary were taken over by the more successful Gnome operation so that later units were badged “Gnome et Rhone” .

RFC ace Albert Ball VC beside a Caudron with Gnome engine

The engine was built under license in several countries including , ironically, Germany!  Two Gnome derived engines were even used on an early Soviet helecopter which flew to a record 2000 feet in 1932 but by this time the rotary and it’s total loss oiling, costly build and the sometimes violent  gyroscopic effect it had on the handling of aircraft it was fitted to (they loved to turn one way but resisted turning the other!) meant the fixed radials and  in-line engines , which could be enlarged and deveoped more easily had overtaken the concept. But what a clever design or the time! One of those impressive pieces of ‘blue sky thinking’ that strike you when you happen across them.

The see how the Monosoupape goes together this brilliant bit if CGI on Youtube is worth a look (  )

To see a one running try this clip on You Tube (  )



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